Hormones play a key role in regulating appetite and hunger. Ghrelin, for example, tells us when we are hungry, and GLP-1 tells us when we are sated and can stop eating. New British research in mice and rats has now found that phenylalanine, an end-product of protein digestion, increases levels of GLP-1 while decreasing ghrelin. In the experiments, a single-dose of phenylalanine reduced food intake by modulating the two hormones, and repeated administration caused weight loss in obese mice and spurred rats to move around more, which could further help weight loss.